A telephone exchange switch allocates calls to an outgoing trunk group consisting of trunk lines to a different exchange switch where a call is to other than a local telephone. Some such calls may have a priority giving them a higher class of service (COS) which allows them to pre-empt calls with a lower class of service. Calls may be allocated to the trunk group regardless of the COS but typically calls such as emergency calls have a dedicated trunk line which ensures that they will be forwarded to the required destination without being routed through an excessive number of exchanges when trunk groups are busy. Clearly if too many trunk lines are reserved for various classes of service the general service will suffer, so the allocation of reserved trunks is an exercise in statistics.
U.S. Pat. No. 6,493,444 to L. Lloyd Williams (published as US 20020071543) and originally filed Mar. 16, 1999 relates to a Public Switched Telephone Network (PSTN) with common channel signaling to a call control node from each switching office, so that node can control the call routing in an efficient manner. This includes detecting calls which require an enhanced level of service, and routing these calls via reserved trunks. It also includes overflowing plain old telephone system (POTS) calls into the reserved trunks under overflow conditions, which tends to reduce or negate the reason for reserved trunks.
U.S. Pat. No. 6,345,092 to Rie Hoshi relates to trunks physically graded into reserved and non-reserved classes of service. At times when utilization is low the specification proposes using non-reserved trunks for all calls, though it may use both reserved and non-reserved without distinction, but when usage is high it places reserved calls onto reserved trunks. The specification also includes restricting all calls when reserved trunks are full, and having grades of reserved call restriction depending on the level of usage of reserved trunks. Because physical trunks are reserved the system must track the usage state of each line within the reserved lines to ensure that it is usable, and the individual allocation of each call to a line.
Tracking and allocating calls under such a system requires fairly high overheads in terms of marking particular lines as reserved and tracking their use and it would be preferable to provide a system where the overheads were reduced.
The present invention provides a solution to this and other problems which offers advantages over the prior art or which will at least provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art in any country.